Form Arrays In Javascript

Why wont this work? checkFields() iF statment is failing. The select must be set to multiple.

<form method="POST" action="" name="this_form" onSubmit="return checkFields();">
<select name="this_select[]" multiple>
<option value="ts1">ts1</option>
<option value="ts2">ts2</option>
<option value="ts3">ts3</option>
<option value="ts4">ts4</option>
</select>
</form>

<script language="Javascript">
<!--
function checkFields(){
var errormsg = "";
if (document.this_form.this_select.value == "") { var errormsg = errormsg + "Please Select an Value.

"; }
if (errormsg != "") { alert(errormsg); return false; }
}
//-->
</script>